Cirrus Logic CS2300-OTP - Manual (Page 13)

CS2300-OTP
DS844F2 13
5.4 Output to Input Frequency Ratio Configuration
5.4.1 User Defined Ratio (R
UD
)
The User Defined Ratio, R
UD
, is a 32-bit un-signed fixed-point number which determines the basis for the
desired input to output clock ratio. Up to four different ratios, Ratio
0-3
, can be stored in the CS2300’s one
time programmable memory. Selection between the four ratios is achieved by the M[1:0] mode select
pins. The 32-bit R
UD
can be expressed in either a high resolution (12.20) or high multiplication (20.12)
format selectable by the LFRatioCfg global parameter.
The R
UD
for high resolution (12.20) format is encoded with 12 MSBs representing the integer binary por-
tion with the remaining 20 LSBs representing the fractional binary portion. The maximum multiplication
factor is approximately 4096 with a resolution of 0.954 PPM in this configuration. See
“Calculating the
User Defined Ratio” on page 22 for more information.
The R
UD
for high multiplication (20.12) format is encoded with 20 MSBs representing the integer binary
portion with the remaining 12 LSBs representing the fractional binary portion. In this configuration, the
maximum multiplication factor is approximately 1,048,575 with a resolution of 244 PPM. It is recommend-
ed that the 12.20 High-Resolution format be utilized whenever the desired ratio is less than 4096 since
the output frequency accuracy of the PLL is directly proportional to the accuracy of the timing reference
clock and the resolution of the R
UD
.
5.4.2 Ratio Modifier (R-Mod)
The Ratio Modifier is used to internally multiply/divide the currently addressed R
UD
(Ratio
0-3
stored in the
register space remain unchanged). The available options for R-Mod are summarized in Table 2 on
page 13 . R-Mod is enabled via the M2 pin in conjunction with the appropriate setting of the M2Config[2:0]
global parameter (see Section 5.7.2 on page 17 ).
Table 2. Ratio Modifier
5.4.3 Effective Ratio (R
EFF
)
The Effective Ratio (R
EFF
) is an internal calculation comprised of R
UD
and the appropriate modifiers, as
previously described. R
EFF
is calculated as follows:
R
EFF
= R
UD
•
R-Mod
Referenced Control Parameter Definition
Ratio 0-3................................ “Ratio 0 - 3” on page 20
LFRatioCfg ............................ “Low-Frequency Ratio Configuration (LFRatioCfg)” on page 20
M[1:0] .................................... “M1 and M0 Mode Pin Functionality” on page 16
RModSel[1:0] R Modifier
00 0.5
01 0.25
10 0.125
11 0.0625
Referenced Control Parameter Definition
Ratio 0-3................................ “Ratio 0 - 3” on page 20
RModSel[1:0] ........................ “R-Mod Selection (RModSel[1:0])” section on page 19
M2Config[2:0]........................ “M2 Pin Configuration (M2Config[2:0])” on page 21